Reduction of RF field disturbance in magnetic resonance apparatus

ABSTRACT

In a magnetic resonance apparatus including a transducer (25, 31 ) positioned in the examination region of the apparatus, the transducer is connected with equipment (49) outside the examination region by way of a lead (47) which is grounded (51, 53) to signals of the frequency of the RF field applied to a subject (5) in the examination region to excite magnetic resonance in the subject, at a distance along the lead from the transducer equal to a quarter wavelength at the RF field frequency (or an odd multiple thereof). Distortion of the RF field due to loading by the transducer is thereby reduced. The transducer may, for example, be a microphone or loudspeaker, e.g. incorporated in an intercom or active noise reduction arrangement, or a sensor for sensing a condition of a patient being examined.

BACKGROUND OF THE INVENTION

This invention relates to magnetic resonance apparatus.

Such apparatus can be used for non-invasive internal examinations ofpatients to produce, for example, cross-sectional internal structuralimages, and blood flow and spectroscopy data.

In use of such apparatus the part of the patient to be examined isplaced in a region of strong static magnetic field to define anequilibrium axis of magnetic alignment in the examination region. Aradio frequency (RF) magnetic field is then applied temporarily to theexamination region, in a direction orthogonal to the static magneticfield direction, to excite magnetic resonance in material, typicallyhydrogen protons, in the examination region. The resulting RF signalsare detected and analyzed. During this sequence of operations one ormore gradients are normally imposed on the static magnetic field in theexamination region, e.g. to encode spatially the detected RF signals, orfor other purposes such as flow encoding.

In use of such apparatus it is sometimes required to dispose atransducer in the examination region, the transducer being connected toequipment outside the examination region by a lead. Examples of suchtransducers are microphones and loudspeakers for communication between apatient being examined and a person, e.g. medical staff outside theapparatus, microphones and loudspeakers of an active noise reductionsystem for reducing disturbance of the patient by noise produced in themagnetic field systems surrounding the patient, and sensors for sensingthe condition of the patient e.g. thermometers, and heart rate andrespiratory monitors.

One problem which arises with such transducers is that they tend to loadthe RF fields produced in the examination region, thereby distorting theRF fields and degrading the data obtained by analyzing the detected RFsignals.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magnetic resonanceapparatus wherein this problem is alleviated.

According to the present invention there is provided a magneticresonance apparatus including a transducer disposed in a region to whichthere is applied in use of the apparatus an RF field for producingmagnetic resonance in a subject for examination by the apparatus, thetransducer being provided with a lead which is grounded to signals ofthe frequency of said RF field at a distance along the lead from thetransducer which is equal to a quarter wavelength at said RF fieldfrequency or to an odd multiple thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

One magnetic resonance apparatus in accordance with the invention willnow be described, by way of example, with reference to the accompanyingdrawings in which:

FIG.1 is a diagram illustrating a patient examination region of theapparatus.

FIG. 2 is a diagram illustrating an earpiece of an active noisereduction headset used in the apparatus.

FIG. 3 illustrates the form of an electrode of a transducer of theheadset; and

FIG. 4 is a diagram illustrating the form of a lead to a transducer ofthe headset.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus, which produces images for medical diagnostic purposes isfor the most part of conventional form.

Referring to FIG. 1, the apparatus includes an electromagnet assembly 1which defines a tubular bore 3 in which, in use of the apparatus, apatient 5 to be examined lies on a patient support 57.

The assembly 1 comprises a main electromagnet which produces a strong,uniform, static magnetic field in the bore 3 in the direction of theaxis of the bore, normally referred to as the z-direction. The assembly1 further includes a gradient coil system whereby a gradient may beimposed on the static magnetic field in the bore 3 in any one or more ofthree orthogonal directions, i.e. x, y and z directions.

The apparatus further includes an RF coil system 7 whereby RF fieldpulses may be applied to at least that part of a patient positioned inthe bore 3 which it is desired to examine. The applied RF field pulsesexcite magnetic resonance in the part of the patient to be examined. TheRF coil system 7 also serves to detect RF signals resulting from theexcited magnetic resonance.

In FIG. 1 the RF coil system 7 is depicted, by way of example, as beingof tubular form and disposed coaxial with the bore 1, surrounding thepatient's head.

Energization of the electronmagnet assembly 1 and RF coil system 7 andprocessing of the detected RF signals to produce an image is effectedunder the control of a computer (not shown) and associated power supplyand control equipment (not shown).

In operation of the apparatus the coil assembly 1 and the RF coil system7 produce a significant amount of noise which can be upsetting to apatient, particularly in the confined space within the RF coil system 7.To reduce this noise to a minimum the patient 3 is provided with anactive noise reduction (ANR) arrangement.

The ANR arrangement includes a headset 9 having two earpieces 11 each inthe form of an enclosure which fits around a respective ear of thepatient 5. The two earpieces 11 are supported on a headband 13 ofspringy plastics material which serves to urge the earpieces 11 firmlyagainst the patient's head.

Referring to FIG. 2, each earpiece 11 includes an outer shell 15 of apolymer-based material which may be loaded, for example, with a materialsuch as glass, carbon, alumina or silica. Within the open end of theshell 15 there fits a plastics foam ring 17 to whose outer end isaffixed a rigid flange plate 19 of plastics material, e.g. apolymer-based material, the plate 19 supporting a plastics foam cushionring 21 which contacts the patient's head.

The enclosure formed by the shell 15 and foam tings 17 and 21 is dividedinto two parts by a perforated polycarbonate grille 23 which ispositioned on the side of the ring 17 remote from the flange plate 19.

A microphone 25 is mounted in the outer part of the enclosure centrallyon the outer side of the grille 23 so as to be spaced from the user'sear. Other positions for the microphone 25 may sometimes be preferablee.g. towards one side of the grille 23, or under the flange plate 19,near the user's ear canal opening. The microphone 25 is protected fromingress of material from outside the earpiece 11 by an impermeablemembrane 27. A permeable, second membrane 29 is provided outside theimpermeable membrane 27 for hygiene reasons, and is changed for eachpatient. The two membranes 27 and 29 are attached at their outerperipheries to the flange plate 19 and may, if desired, be arranged tocover the cushion ring 21 for additional hygienic protection.

The microphone 25 is non-magnetic, that is to say, it is of a kind whichdoes not utilise a magnetic field for its operation, and is otherwiseconstructed using substantially only non-ferromagnetic materials. Tothis end the microphone 25 is suitably of an electret type, but mayalternatively be of a piezoelectric or condenser type. One suitableelectret microphone is that described in the Journal of the AcousticSociety of America, Vol 40 (1966), part 6, pages 1433 to 1440, but withferromagnetic materials removed or, where necessary, replaced bynon-ferromagnetic material. In particular, the steel case is replaced bya brass case.

The inner part of the earpiece enclosure houses a loudspeaker 31 whichis also non-magnetic e.g. a condenser, electret or piezoelectricloudspeaker. In FIG. 2 the loudspeaker 31 is depicted as a piezoelectricloudspeaker which has the advantages that it requires a relatively lowdrive voltage and no bias voltage. The loudspeaker 31 includes a film 33of piezoelectric plastics material, for example polyvinylidene fluoride(PVDF), or a co-polymer of PVDF and trifluoroethylene, which is clampedaround its periphery between two stiff polymer tings 35, suitably madeof polycarbonate, the rings 35 being secured within the shell 15, andbeing provided with a by-pass hole 37, to equalise air pressure on thetwo sides of the film 33. The film 33 is provided on each side with anelectrode 39 of non-ferromagnetic electrically conductive material, e.g.copper or aluminium. To reduce the possibility of eddy currentscirculating in the electrodes 39, which can arise due to the rapidchanges in the RF and gradient magnetic fields during an imagingsequence, each electrode 39 is divided by suitably positioned slots 41into a series of narrow parallel strips 43, electrically connectedtogether at one end, as illustrated in FIG. 3.

When an alternating potential is applied between the electrodes 39 ofthe film 33, the material of the film 33 expands and contracts in theplane of the film 33. Foam 45 placed behind the film 33 causes the filmto dome, as shown in FIG. 2. As a result of this, the expansion andcontraction of the film material is converted to a pistophonic motion togenerate sound waves directed towards the user's ear corresponding tothe applied drive voltage.

Electrical connection to the microphone 25 and loudspeaker 31 of eachearpiece 11 is made by way of leads 47 (see FIG. 1) which pass throughthe shell 15 via grommets (not shown), the leads 47 each being in theform of a twisted pair of wires (not shown) of non-ferromagnetic metalsurrounded by a braid screen (not shown) of non-ferromagnetic metal.

It will be appreciated that by virtue of the complete absence ofmagnetic material in the ANR headset 9, the presence of the headset doesnot significantly interfere with the homegeneity of the static magneticand the RF fields in the examination region of the magnetic resonanceapparatus. Hence, the presence of the headset 9 does not degrade imagesproduced by the apparatus.

In accordance with the present invention, in order to prevent imagedegradation by virtue of the headset 9 loading the RF fields in theapparatus, the leads 47 connect the microphones 25 and the loudspeakers31 to a signal processor 49 of the ANR arrangement by way of a falterunit 51 and a virtual earth arrangement 53.

The processor 49 utilises the signals produced by the microphones 25 toproduce signals for application to the loudspeakers 31 of such a form asto minimise the noise heard by the patient 1. Thus the signals arerequired to be of such a form that the loudspeakers 31 produce at thepatient's ears sound waves of the same amplitude and frequency contentas, but of opposite phase to, the noise sound waves reaching thepatient's ears.

Referring to FIG. 4, the filter unit 51 includes twelve series resonantcircuit filters 55 tuned to the frequency of the RF fields applied tothe examination region in use of the imaging apparatus, there being onefilter 55 for each wire and the screen of each lead 47. Each filter 55is connected between ground and a point on the associated wire or screenwhich is at a distance from the associated ANR headset transducer, i.e.microphone 25 or loudspeaker 31, equal to a quarter of a wavelength ofwaves of the RF field frequency propagating along the transducer lead47. Each wire and screen is thus effectively grounded to RF fieldfrequencies at this point and consequently presents a high impedance tocurrents at the RF field frequency at its end adjacent the associatedANR transducer 25 or 31. Hence the presence of the transducers 25 and 31in the imaging apparatus examination region interferes with, i.e. loadsthe RF fields produced for imaging to a negligible extent. It will beunderstood that the passage of audio frequency signals along the loads47 for noise reduction is not affected.

Where, as is usually the case, a suitable ground connection is notavailable inside the bore 3 of the imaging apparatus (the bore 3typically being internally lined with a plastics material cowling), thevirtual earth 53 is required. This comprises a line of length equal to aquarter wavelength at the RF field frequency or to an odd multiplethereof, with an open circuit at its end remote from the associatedfilter 55. The line suitably comprises a serpentine track formed on oneside of a plastics material sheet. The serpentine track may suitably beformed on the plastics material bore cowling, where possible.

It will be understood that whilst in the magnetic resonance apparatusdescribed above, by way of example, the ANR transducers are mounted in aheadset worn by the patient, this is not necessarily the case. In otherapparatus according to the invention incorporating an ANR arrangement,the ANR arrangement transducers may be otherwise supported, e.g. atappropriate places on the RF coil system 7, or other part of themagnetic resonance apparatus per se.

It will also be understood that he technique used for the leads 47 ofthe microphones 25 and loudspeakers 31 to minimise RF field disturbancemay be used with transducers required to be positioned in theexamination region of a magnetic resonance apparatus, other than ANRarrangement transducers. Examples of such other transducers aremicrophones and loudspeakers for communication between a patient andmedical staff, and sensors for sensing the condition of a patient e.g.thermometers, and heart rate and respiratory monitors.

We claim:
 1. A magnetic resonance apparatus for examining a subject,comprising:a) a region in which the subject is positioned duringexamination; b) radio frequency means for applying a radio frequencyfield having a frequency to the region for producing magnetic resonancein the subject; and c) a transducer disposed in the region and having alead which is grounded to signals having the frequency of said radiofrequency field at a distance along the lead from the transducer whichis equal to a quarter wavelength at said radio frequency field frequencyor to an odd multiple thereof.
 2. The apparatus according to claim 1,wherein said lead is grounded by way of a filter tuned to saidfrequency.
 3. The apparatus according to claim 1, wherein there are aplurality of leads, each lead including a conductor, and each conductorbeing separately grounded.
 4. The apparatus according to claim 3,wherein each lead includes a screen, and each screen is separatelygrounded.
 5. The apparatus according to claim 1, wherein said lead isgrounded to a virtual earth.
 6. The apparatus according to claim 5,wherein said virtual earth includes an open circuit terminated line oflength equal to a quarter of a wavelength at said radio frequency fieldfrequency or to an odd multiple thereof.
 7. The apparatus according toclaim 6, wherein said line includes a track formed on a cowling ofplastics material which surrounds the examination region.
 8. Theapparatus according to claim 1, wherein said transducer forms a part ofan active noise reduction arrangement for use by a human subject beingexamined in the apparatus.
 9. The apparatus according to claim 1,wherein said transducer is an electro-acoustic transducer for use by ahuman subject being examined in the apparatus for communication with aperson outside the apparatus.
 10. The apparatus according to claim 1,wherein said transducer is a sensor for sensing a condition of a humansubject being examined in the apparatus.
 11. The apparatus according toclaim 1, wherein said transducer is an acoustic transducer.
 12. Theapparatus according to claim 1, wherein said transducer is anon-magnetic type transducer.
 13. The apparatus according to claim 12,wherein said transducer is a piezoelectric loudspeaker having a film ofpiezoelectric plastics film carrying an electrode on each main surfaceand being supported within an enclosure in a domed condition.